368 



NATURE 



[December 2, 1915. 



various operations which result finally in the. 

 finished map or plan. A valuable feature of the 

 book is that the accuracy attainable in an opera- 

 tion is clearly stated, and the surveyor is taught 

 to plan his work according to the accuracy which 

 the purpose that it is to serve demands, while not 

 attempting such a degree of precision as would 

 lead to an expenditure of time and money which 

 would not be economical under the conditions 

 existing. In this connection a little more might, 

 perhaps, be said on the selection of the scale of 

 maps as determined by the permissible errors of 

 the work and the limitations of the draughtsman. 

 A short account of the theory of errors and its 

 application to survey measurements would also 

 seem to fall within the scope of this text-book. 



The theodolite and its adjustments are fully 

 dealt with as regards vernier instruments, since 

 those fitted with micrometer microscopes are con- 

 sidered to belong to the more precise work of 

 higher surveying. At the same time, it may be 

 questioned whether the simpler forms of micro- 

 scope which are in use abroad are not both more 

 easy to read and less liable to injury than the 

 vernier. 



The execution of a small triangulation, in 

 which plane trigonometry only is required, fur- 

 nishes an instructive example of the various opera- 

 tions involved, base measurement, selection of 

 stations, and measurement of angles, and these 

 are discussed with special reference to the errors 

 which are liable to occur, and the limits within 

 which they may be expected to fall. 



Levelling with the spirit-level is dealt with in 

 the same detailed manner, and a simple formula is 

 given to guide the leveller in judging of the 

 accuracy of his work. 



In the chapter on the calculation of areas, the 

 more modern patterns of planimeter might be 

 mentioned, since with them the instrumental 

 error can be eliminated from the result by 

 changing the position of the fixed point and re- 

 peating the measurement ; also a convenient check 

 on the accuracy of the measurement is provided. 

 A reference to the slide rule, and to computing 

 machines, computing table, etc., would be of 

 value as indicating to the surveyor ways in which 

 he may save time in his work. 



The tacheometer is not mentioned, since it is 

 probably considered as more suited for topo- 

 graphical surveys, but in many countries it is 

 largely employed in preliminary surveys and in 

 much large scale work. 



Theoretical accounts of most operations are 



given, and these might even be extended with 



advantage, since they indicate clearly the 



directions in which errors are to be anticipated, 



NO. 2405, VOL. 96] 



and the conditions under which they tend to in- 

 crease in magnitude. 



The book is well illustrated and contains 

 numerous practical examples, which have been 

 well chosen with the view of showing such cases 

 as may be expected to occur in ordinary practice. 



H. G. L. 



OUR BOOKSHELF. 



Unit Photography. By F. M. Steadman. Pp. xi 

 + 160. (London: Constable and Co., Ltd., 

 1914.) Price 85. 6d. net. 

 The author deplores the " whole train of lament- 

 able conditions in photography relative to ex- 

 posure," and sets himself "to establish a rational 

 scientific foundation for the practice of photo- 

 graphy and for the study of light as it is daily 

 observed in nature." He says that the apertures 

 of lenses, the sensitiveness of emulsions, and the 

 chemical energy of lights lack simple units of 

 measurement, and that therefore photography is 

 not scientifically practised. He adopts f/6/\. as 

 the unit for lens apertures, calling the figures 

 which represent the intensities of the ordinary 

 apertures up to //i the "cone unit values." His 

 standard of "actinicity " is "that rate of emission 

 which will produce a least visible tint in one 

 minute (or 64 seconds) when the convergence is 

 //i (or 4000 cone units)." This is an "actino.' 

 "The speed of an emulsion is defined as the time 

 required for it to suffer an effect which is known 

 as normal exposure, when it is exposed to a sur- 

 face having an intensity of one actino and 

 through a diaphragm having a convergent value 

 of one cone unit." He gives full instructions for 

 the use of these standards, and works out a 

 number of problems that will prove of much in- 

 terest to the curious. His "//i actinometer " is 

 a box which has an opening equal in diameter to 

 its distance from the opposite side, where a piece 

 of sensitive paper is placed. The first experi- 

 ment suggested in " unit actinometry " is to 

 measure or estimate the average diameter of a 

 flame, put the sensitive paper at a distance of two 

 diameters {f/2) from it, and time the period neces- 

 sary to produce the first visible effect. The calcu- 

 lation as explained gives " the average intensity 

 of the flame in actinos." 



The volume contains useful suggestions, though 

 we cannot see that the methods propounded are 

 any more " scientific " than the ordinary procedure 

 with commercial exposure meters, and we dis- 

 tinctly differ from the author in his idea as to the 

 "lamentable condition" of photography with re- 

 gard to exposure, so far as this country is con- 

 cerned. It may be difi"erent in America. 



The Student's System. By V. Russell. Pp. 113. 



(London: J. M. Dent and Sons, Ltd., 1915.) 



Price IS. 6d. net. 

 This book advocates the use of a loose-leaf note- 

 book by University students, and by all who are 

 engaged in study or research. It is claimed that, 

 by adopting this system, a student need not carry 



